1. Field of the Invention
The present invention relates to a black and white photothermographic material and an image forming method. More particularly, the invention relates to a high image quality black and white photothermographic material for medical use and an image forming method using the same.
2. Description of the Related Art
In recent years, in the medical field and the graphic arts field, there has been a strong desire for providing a dry photographic process from the viewpoints of protecting the environment and economy of space. Further, the development of digitization in these fields has resulted in the rapid development of systems in which image information is captured and stored in a computer, and then when necessary processed and output by transmitting it to a desired location. Here the image information is output onto a photothermographic material using a laser image setter or a laser imager, and developed to form an image at the location. It is necessary for the photothermographic material to be able to record an image with high-intensity laser exposure and that a clear black-tone image with a high resolution and sharpness can be formed. While various kinds of hard copy systems using pigments or dyes, such as ink-jet printers or electrophotographic systems, have been distributed as general image forming systems using such digital imaging recording materials, images on the digital imaging recording materials obtained by such general image forming systems are insufficient in terms of the image quality (sharpness, granularity, gradation, and tone) needed for medical images used in making diagnoses, and high recording speeds (sensitivity). These kinds of digital imaging recording materials have not reached a level at which they can replace medical silver halide film processed with conventional wet development.
Photothermographic materials utilizing organic silver salts are already known. Generally, the photothermographic material has an image forming layer in which a reducible silver salt (for example, an organic silver salt), a photosensitive silver halide, and if necessary, a toner for controlling the color tone of developed silver images are dispersed in a binder.
Photothermographic materials form a black silver image by being heated to a high temperature (for example, 80° C. or higher) after imagewise exposure to cause an oxidation-reduction reaction between a silver halide or a reducible silver salt (functioning as an oxidizing agent) and a reducing agent. The oxidation-reduction reaction is accelerated by the catalytic action of a latent image on the silver halide generated by exposure. As a result, a black silver image is formed on the exposed region. There is much literature in which photothermographic materials are described, and the Fuji Medical Dry Imager FM-DPL is an example of a medical image forming system that has been made commercially available.
Photothermographic materials using a silver salt of a nitrogen-containing heterocyclic compound as an organic silver salt and a hydrophilic binder such as gelatin are disclosed in U.S. Pat. No. 6,576,410.
On the other hand, attempts have also been made at applying the above-mentioned black and white photothermographic material as photosensitive material for photographing. The photosensitive material for photographing as used herein means a photosensitive material on which images are recorded by a one-shot exposure through a lens, rather than by writing the image information by a scanning exposure with a laser beam or the like. Conventionally, photosensitive materials for photographing are generally known in the field of wet developing photosensitive materials, and include films for medical use such as direct or indirect radiography films, mammography films and the like, various kinds of photomechanical films used in printing, industrial recording films, films for photographing with general-purpose cameras, and the like. For example, an X-ray photothermographic material coated on both sides using a blue fluorescent intensifying screen described in Japanese Patent No. 3229344, a photothermographic material containing tabular silver iodobromide grains described in Japanese Patent Application Laid-Open (JP-A) No. 59-142539, and a photosensitive material for medical use containing tabular grains that have a high content of silver chloride and have a (100) major face, and that are coated on both sides of a support, which is described in JP-A No. 10-282606, are known. However, while high sensitivity is required for photographing use and even higher sensitivity is especially required for recording X-ray images so as to reduce an amount of radioactive radiation exposure with respect to the human body, photothermographic materials are still far from reaching a level of sensitivity that satisfies such requirements. In photothermographic materials in which the amount of coated silver of photosensitive silver halide is limited in view of film turbidity and storability, enhancement in sensitivity of a silver halide grain itself is strongly demanded.